The present invention is generally related to illumination for imaging systems and is specifically related to dark-field illumination for microscopic and macroscopic imaging systems.
For the present invention, imaging systems for small objects including microscopes and macroscopic imagers, are of particular interest. Generally, microscopic systems have a magnification between 10 and 1000 where the magnification is defined by the ratio of the image size compared to the object size in linear dimension. Macroscopic systems can have images that are magnified between 0.3 and 10 times. Although a xe2x80x9cmagnificationxe2x80x9d value less than 1 is really a reduction where the image produced is smaller than the object thereof, we still use the word xe2x80x9cmagnificationxe2x80x9d. The present invention can be employed on both systems with some benefits being particular to macroscopic systems.
The orientation of an illumination source of an imaging system can dictate the way that the object being viewed will appear to a viewer. Generally, an object is viewed in either of two common configurations. A first is known as xe2x80x9cbright-fieldxe2x80x9d illumination where the image background is bright and features of the object being viewed are dark. Bright-field illumination is particularly useful for translucent type objects. The effect is achieved by backlighting the object as is shown in prior art drawing FIG. 1. A second illumination configuration produces an image known as a xe2x80x9cdark-fieldxe2x80x9d image. An object 6 resting on a support 4 can be illuminated with light from a light source 7 and reflector 5 that is outside the field-of-view 2 of the imaging optics 1. Light 9 reflected from the object features then can enter the pupil of the imaging optics 1 to form an image where the features of the object are light and the image background is dark as the illumination source can not be seen by the imaging system.
There are several ways to provide for illumination sources that produce a dark-field image. The most common is to arrange the illumination source to shine light onto the object from the side in a direction that is transverse with respect to the optical axis of the system. Any light from the source that does not get reflected from the object will continue to propagate past the object but will not enter the pupil of the imaging optics as is shown in the prior art drawing of FIG. 2. In this way, only light from the object will enter the imaging optics and form a bright image of the object features against a dark background. A problem with this system is that the light that is scattered from the object must be scattered at a large angle in order to enter the imaging optics. The efficiency of high angle scattering is low. For light sufficient in quantity to form a usable image to be scattered by an object at a large angles requires that the source be very bright. Bright sources in microscopes are undesirable because of heat and other problems.
Alternatively, it is possible to arrange a light source to illuminate the object from a direction that forms an acute angle with, the system""s optic axis. An example in the art that is noted in the U.S. Pat. No. 4,906,083, suggests that a central zone of the field-of-view be blocked such that the illumination source can not be viewed directly by the imaging optics. In this case, slight changes in the propagation angle of light rays from interaction with the object will divert those rays into the pupil of the imaging optics. This is illustrated in prior art drawing FIG. 3 where a light stop 8 is placed between the microscope objective lens and the source so that light from the source can not pass into the imaging optics without first being reflected from the object. Light rays 3 that are propagating in a direction that would cause them to enter the pupil of the imaging optics are blocked. Light rays being reflected only slightly at the object 6 can enter the imaging optics to form an image. If light ray 9 did not interact with the object, its path would lead to a point outside the entrance pupil of the objective lens 1. In this way, very slight reflections from the object will be coupled to the image beam. Alternatively, light rays can interact with features 19 that are inside of a translucent object and thereby contribute to the object beam. For objects such as gems where the surface quality is of concern, this method may be very useful.
A major problem with the arrangement of FIG. 2 is that strong glare generated at smooth flat surfaces can hide fine image features. It is the subject matter of Jasqur that addresses the reduction of this surface glare. xe2x80x9cForward scatteringxe2x80x9d where the change in the angle of propagation is small, is efficient with respect to the quantity of light in the scattered beam. Unfortunately, this can be a disadvantage when looking at objects that may have interesting features that can be hidden by the large amounts of surface scattered light. If one is not interested in viewing a surface of the object but detailed features thereon, the surface scattered light can hide those details. The illumination system of FIG. 1 could be preferred to see the features of a surface; the system of FIG. 2 may be preferred for seeing surfaces. Glare is generated in reflections from a smooth surface. The tendency is for light to become linearly polarized in a direction that is parallel to the surface that reflects the light. This is a result of the Fresnel equations and the bias-of a surface to more strongly reflect light that has its polarization along the plane of the reflecting surface. If the subtle features of an object reflect light in small amounts, the strong light reflected from a nearby surface, the glare, can overwhelm the imaging of those features. To eliminate the glare, one could discriminate against the light that tends to take the polarization of the surface with a polarizing filter or analyzer. It is an oversight of Jasgur in lines 48-51 of the first column to suggest that the two polarizers simple must be crossed to eliminate the glare. It is not enough to simply cross polarizers to reduce the glare from a surface.
A designer can produce many variations of light source arrangements that avoid shining source light directly into the pupil of the imaging optics. In all cases of the prior art, a geometrical arrangement is used whereby the light from the source propagates in a direction such that it will not enter the pupil of the imaging optics. Although dark-field illumination methods are known in microscopic imaging arts, the methods of attaining a dark-field have been problematic as the quality of the dark-field has been limited and the complexity of the optical arrangements required have been undesirable and have resulted in secondary problems such as glare. The use of polarizing microscopes to produce a dark-field image may have been attempted, but the results are inferior to the oblique illumination methods. Because microscopes in general require very bright sources, the use of polarization techniques which require even brighter sources as fifty percent of the light can be lost at a first polarizer, the prior art teaches away from the use of polarization techniques for dark-field illumination systems. Currently, the use of the oblique method is the preferred method in the art.
The use of other optical elements and devices in the optical paths of microscopic systems have been practiced in the arts. Particularly, the use of cameras and polarizers are common. A camera coupled to the system can provide the user relief from the concentration associated with looking into a microscope eyepiece. Cameras are also useful for recording image features for study at a later time or for publishing purposes. As the eyepiece of a microscope is generally designed for a single viewer, a camera is useful for group viewing. Both still photographic type cameras and electronic video cameras are currently in widespread use in conjunction with microscope systems.
Polarizing elements can be used to serve many functions in microscope imaging techniques. Polarizing microscopes use polarizers to look at anisotrophic materials, remove glare, control illumination intensity, multiplex illumination beams of various colors or other properties, and other special effects.
In U.S. patent prior art class/subclass 359/386 we find the illumination systems for microscopes that use polarized light. In U.S. Pat. No. 3,567,309, inventor Jasgur teaches the use of polarizers to limit glare. The detailed description shows where Jasgur has left a very important omission that results in a great improvement in removal of the glare generated at surface reflections from optical element surfaces. U.S. Pat. No. 4,515,445 provides for an improvement in image contrast by a double pass through the object. In lines 41-44 of column 3, we learn that polarizers can be used in the path of the illumination system to suppress reflections. This may be the most common use of the polarizers in an optical system and is important in microscopy where reflections can otherwise hide very subtle features.
In lines 34-37 column 1, U.S. Pat. No. 4,906,083 authors Spencer and Frank remind us that: xe2x80x9cdark-field illumination is used, in which the object is illuminated from the side or the central zone of the light beam is screened out.xe2x80x9d The subject of the patent is directed at a dark-field illumination system using polarization means. As this is the very object of the present invention, it is advised that the reader becomes familiar with the above referenced patent ending in ""083. The inventor discovered that a dark-field could be provided with a very bright source and provided a novel means to deliver the light to the system. Messrs. Reinheimer and Klein present an arrangement in U.S. Pat. No. 3,572,885 which includes a Bertrand lens. The disclosure of U.S. Pat. No. 4,634,240 is especially important with respect to the present invention in that it employs the combination of a video camera in conjunction with a polarizing means to control illumination. It will be understood from the detailed description that it is also the intention of this invention to control illumination with a similar combination. The present invention distinguishes itself from the invention of Suzuki in that the polarizer is used to control the quantity of light that enters the camera or for an exposure control means. The polarizers of the invention are used to discriminate against light having a certain property and does not act uniformly on the entire beam as the polarizers of the Suzuki invention do. An incident light, bright-field Kohler device described in Stankewitz et al, U.S. Pat. No. 4,386,830, provides illumination for microscopes with polarized light. Finally, Dalbera in U.S. Pat. No. 3,822,926 teaches a particularly clever means to correct the xe2x80x9cMaltese crossxe2x80x9d known to appear in systems using polarizers. The invention of Dalbera will improve the effectiveness of many systems that employ polarizers.
Although we find considerable work in the field, the inventions of the art do not and can not provide the advantages found in the current invention. The current invention provides for a variable dark-field illumination system using polarizers in combination with a camera.
While the systems and inventions of the current art are designed to achieve particular goals and objectives, some of those being no less than remarkable, these inventions have limitations and faults that prevent their use to achieve the benefits herein described. The inventions of the art do not, and can not, be used to realize the advantages and objectives of the present invention.
Comes now, an invention of a variable dark-field illumination system for micro and macro optical imagers including devices and methods.
A fundamental difference between the illumination systems of the invention and that of the prior art can be found in the use of new geometries of optical elements which produce the variable dark-field effect. The systems of the invention are particularly characterized in that the light from the light source propagates in a direction that is substantially parallel to the optical axis of the imaging system. Previous dark-field illumination systems require geometric arrangements of the light source such that the light would not propagate in a direction that would enable it to pass into the pupil of the imaging optics or require prohibitively bright sources. In the system of the invention, light from the source that is propagating directly toward the pupil can be stopped at a polarization analyzer if that light has not interacted with the object being viewed. These new geometries can include dark-field object space of infinite depth which is particularly useful in macroscopic imaging systems which can have object depths that are far greater than the depths that are found in microscopic systems. This large depth dark-field of the invention can allow unique optical arrangements of other optic elements that may be limited by systems of the art. Methods of the art generally used certain geometric arrangements of illumination sources to prevent light from the illumination source from directly entering the pupil of the imaging optics.
It is also a fundamental difference of the present invention compared to the prior art to provide the possibility of a variable dark-field. A variable dark-field effectively allows one to adjust the light levels of the light emanating from the object before the light enters the imaging optics such that the contrast range of the object""s interesting features can be optimized with respect the range in which the detector is linearly sensitive thus giving maximum contrast at the final image. The prior art is limited to a dark-field which has a dark level that is not adjustable. The illumination source geometry is generally fixed in a way that prevents one from adjusting the level of the darkest object features without changing the brightest image features. A variable dark-field can be used to match the image signal contrast to the dynamic range of a detector thereby providing the highest possible contrast in a final image. The detector can be a human eye, photographic film, or electronic type cameras such as CCD cameras; each having a different dynamic range.
The present invention provide a more complete solution to a problem which was recognized in the art, specifically, in U.S. Pat. No. 4,906,083 where the solution proposed is a stronger light source. Simply increasing the strength of the illumination source will adjust the intensity bias but has no effect on the image signal contrast. The inventors noticed the phenomenon when experimenting with a particular detector and a new illumination scheme. By using a video camera designed for low light conditions, it was found that a great improvement in final image contrast could be attained with a variable dark-field as opposed to the absolute dark-field of the art. This was due to the subtle transitions from the linear to non-linear regions of the response curve of the human eye where the effect is not so strong. Nature has provided the eye with a very sophisticated image processing system that tends to adjust the dynamic range of the eye to match the object being viewed, thus hiding the effect from most observers. Without the benefit of the low light level camera 14 where the new effect is more clearly observed, the effect has remained hidden. Users of microscopes equipped with polarizers have manipulated light sources and the light from the object to produce many effects that are useful, however, until the inventor applied the variable dark-field method to a video camera detector, the usefulness of a variable dark-field could not be recognized with devices of the art.
Accordingly, it is a primary object of the invention to provide an illumination system for micro and macro imaging systems. It is also an object of the invention to provide a system of variable dark-field illumination for micro and macro imaging systems. It is an object of the invention to provide an improved means of viewing macroscopic objects as a result of the new geometrical arrangement of optical elements including lenses, light sources, polarizers, objects, video cameras and others. It is a further object of the invention to provide a new and novel system to remove the illumination source from the field-of-view of a microscope.
A better understanding can be had with reference to the detailed description of preferred embodiments and with reference to the appended drawings. These embodiments represent particular ways to realize the invention and are not inclusive of all ways possible to benefit from the invention. Therefore, there may exist embodiments that do not deviate from the scope of this disclosure as set forth by the claims, but do not appear here as specific examples.